This paper aims to characterize and reduce the level of nitrogen monoxide (NO) during the combustion of rapeseed oil methyl ester (ROME) in a semi-industrial boiler. First, the formation of NO is characterized during transitional and steady-state operation of boiler; the influence of combustion pressure, excess air, exhaust gas temperature, spray cone angle, and combustion air swirl angle on the level of NO is evaluated, suitable burner operating points for control of NO are recognized, and contributions of thermal and prompt NO to the total level of NO are obtained. At the next level, the potential of air-staging technique (injection of extra air) in the reduction of NO is studied. Results reveal that the level of NO rises significantly in the post-flame zone 10 cm after the tail of the flame, where only the formation of thermal NO is probable in the chamber. Staged air is able to reduce the level of NO up to 10% without any negative impact on the operation of the boiler. Results also reveal that fuel spray pattern and air dynamic are able to reduce the level of NO during the combustion. The level of NO at a swirl angle 45 degrees is, on average, 13%, 10%, 7%, and 16% lower than that at swirl angles of 30 degrees, 37.5 degrees, 52.5 degrees, and 60 degrees, respectively. Pulverization of biodiesel at a 60 degrees pattern leads to the formation of 6%, 3%, and 10% less NO, compared with that at 30 degrees, 45 degrees, and 90 degrees patterns, respectively. Thermal NO accounts for similar to 80% of total NO during the steady-state efficient combustion (combustion pressure of >19 bar, and equivalence ratio congruent to 0.8 (excess air 25%)) and prompt NO contributes more than thermal NO to the total level of NO during the burner startup.